Patent GB-664210-A (GB)
Title: Improvements in or relating to electric motor control systems
IPC: H02P7/26
Filed: 1949-08-05
Abstract: 664,210. Control of D.C. motors. WESTING- HOUSE ELECTRIC INTERNATIONAL CO. Aug. 5, 1949. [Aug. 10, 1948] No. 20416/49. Class 38 (iii). [Also in Group XXXV] A direct-current motor has an armature and two separate series field windings, the motor being energised from an alternating-current source, and the control system comprises a power supply transformer having two secondary windings each having two end points and an intermediate tap point, said tap points being connected to respective ones of said field windings, two pairs of rectifier tubes having a common cathode lead connected to said armature, the anodes of each pair of rectifier tubes being respectively connected to the end points of a separate one of said secondary windings, control circuits for the respective pairs of tubes and adjustable control means with which said control circuits are associated in mutually inverse relation so that a change in adjustment of said control means causes increased conductance of one pair of tubes while the conductance of the other pair is reduced. An electric motor has an armature 7 and opposing series field windings 8, 9, the direction of motor rotation being dependent upon whether thyratrons 11, 12, or 13, 14, pass the heavier current. The magnitude of rectified current supplied to the field winding 8 through valves 11, 12, is dependent upon the magnitude of variable voltage across a resistor 37, that passed to field winding 9 through valves 13, 14, being in accordance with the variable voltage across resistor 42. The resistors.37, 42, are connected in the respective anode circuits of a twin triode valve 52 which is controlled by another twin triode valve 61. A motor-driven tachometer generator 69, together with a selected portion of a speed control rheostat 70, are connectable in the grid control circuits of the valve 61. The selected setting of contact 72 of rheostat 70 controls the direction and speed of the motor. When contact 72 is at the mid-point the field windings 8 and 9 carry equal current and the motor does not rotate. Movement of the slider 72 away from the neutral position results in heavier current in one field winding accompanied by reduced' current in the other, the motor developing a torque. The generator 69 generates a voltage proportional to its speed which opposes the reference voltage determined by the setting of slider 72 on potentiometer 70, deviations from the desired speed are thereby corrected by appropriate control of the main rectifying valves. A voltage is developed across resistor 89 which is dependent upon the motor current and, if this current exceeds a predetermined value, a tube 90 breaks down. A tube 94 then passes current through resistors 37 and 42 to change the firing-angles of the main rectifying valves in a sense to reduce the motor current. Capacitors 85, 86, prevent sudden changes of the voltages across resistors 37, 42, when the motor is being accelerated from rest, thus providing smooth starting operation. When the motor is to be stopped by actuation of contact 83 relay CR1 drops immediately, the relay CR2 remaining energised for an additional period. During that interval the voltage of the tachometer generator 69 is alone effective to control the valve 61, thus, while the motor is still running in its original direction, the field winding for reversal torque is predominantly energised. The motor is thus regeneratively braked, the main rectifying valves acting as inverters. The subsequent dropping of relay CR2 disconnects the motor from the supply. Regeneration may be also obtained during reversing operation, the tap 72 being moved from one to the other side of the zero position so that the motor will continue to run for a period in a direction opposed to that for which it is excited. A similar temporary regenerative operation is obtained when the control rheostat is moved to reduce the speed, or when the tachometer voltage exceeds the reference voltage due to overspeeding.